Automotive and commercial vehicles include a powertrain that is comprised of an engine, a multi-speed transmission, and a differential or final drive. The multi-speed power transmission requires a supply of pressurized fluid to properly operate. The pressurized fluid may be used for such functions as cooling, lubrication, and torque converter operation. It is well known that the lubricating and cooling capabilities of transmission oil systems greatly impact the reliability and durability of the transmission. Additionally, multi-speed power transmissions require the hydraulic system to provide controlled engagement and disengagement, on a desired schedule, of the various torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement.
Transmissions are traditionally supplied with hydraulic fluid by a wet sump (i.e., single-pump, internal reservoir), cascading oil system, which is separate from the engine oil system. The fluid is typically stored in a main reservoir or main sump volume where it is introduced to a pickup or inlet tube for communication to the single hydraulic pump. The pump operates to pressurize the fluid for subsequent communication to the transmission.
It is well known to utilize one or more fixed displacement (or “PF”, according to industry custom) pumps in multi-speed transmissions. A PF pump can generate relatively instantaneous pressure and flow to a hydraulic circuit when the circuit is opened due to the positive displacement characteristic of PF type pumps.
A high-pressure PF pump serves a number of important roles. First, high oil pressure has traditionally been required to maintain torque converter charge pressure, which is the torque converter oil inlet pressure at the centerline of the transmission. It is necessary to maintain this pressure to avoid cavitation, which is not only inefficient, but can be damaging to the internal componentry of the torque converter. More notably, the high-pressure pump supplies the fluid pressure necessary to selectively apply the transmissions torque transmitting mechanisms. The single PF pump is required to satisfy both the high-pressure/low-flow requirements while simultaneously satisfying the low-pressure/high-flow requirements. The single PF system is inefficient in applications requiring high-pressure fluid because the PF pump continues to deliver high flow, high pressure fluid even when only low pressure and/or low flow is needed. Hence, the single PF pump is expending power equal to its total output flow and pressure (i.e., high-pressure and high-flow), even though the transmission is using only a portion of that flow. This parasitic loss results in unnecessary consumption of power from the motor vehicle engine or battery and may tend to reduce the overall life of the pump.
In systems where the hydraulic circuit has a relief valve, the valve will “dump” excess fluid flow as it is being pumped by the PF pump, thereby generating undesirable heat in the hydraulic fluid. Multiple PF pump systems are designed to minimize this undesirable behavior by having a different PF pump that can be called upon to deliver a required fluid flow or pressure depending upon the operating conditions. However, PF systems with multiple pumps still have the high pressure deficiencies mentioned above.
A common type of PF pump used in multi-speed transmissions is a gerotor pump (“GP”). The GP includes a ring gear supported by a pump housing. In addition, a pinion gear is rotatably mounted inside the ring gear for rotation about parallel, laterally separated centerlines. The teeth on the respective gears cooperate to define a plurality of variable volume pumping chambers. During rotation of the gear members, each pumping chamber expands in an inlet half, and collapses in a discharge half. Fluid from the GP's low-pressure, inlet port is drawn into pumping chambers that are increasing in volume. Upon further rotation, when the pumping chambers are decreasing in volume, the fluid is pushed out through the pump's outlet or discharge port at a higher pressure. The inlet and the discharge ports are separated angularly or “timed” to prevent the pump chambers from simultaneously overlapping both the inlet port and the discharge port.
It is also known to use a single variable displacement (or “PV”, according to industry custom) pump to satisfy the hydraulic fluid needs of a multi-speed transmission. The PV pump produces a variable flow on demand. Thus, in standby conditions, PV pump systems do not circulate as much hydraulic fluid.
A single PV pump traditionally employs a rotor having multiple slots circumferentially disposed about the rotor, wherein a plurality of vanes are slidably disposed, and a slide member to vary the volume of fluid delivered to a hydraulic work circuit. The slide member is eccentrically offset from the rotor to create fluid chambers defined by the vanes, rotor, and inner surface of the slide. A compression spring, connected to a regulator arm on the periphery of the slide, positions the slide to create large fluid chambers as the default. When the transmission requires less fluid, a pressure regulator directs fluid from the pump output line to a regulating chamber in the PV pump. Pressure in the regulating chamber pivots the slide against the force of the spring to more closely align the centers of the rotor and slide, reducing the eccentricity offset, thereby reducing the size of the fluid chambers. This reduces the amount of fluid drawn into the pump from the fluid reservoir and likewise, the amount of fluid output by the pump.